Common pollutants arising from the use of internal combustion engines are nitrogen oxides (commonly denoted NOx) and particulate matter (also known simply as “soot”). NOx is generally associated with high-temperature engine conditions, and may be reduced by use of measures such as exhaust gas recirculation (EGR), wherein the engine intake air is diluted with relatively inert exhaust gas (generally after cooling the exhaust gas). This reduces the oxygen in the combustion regime and reduces the maximum combustion temperature, thereby deterring NOx formation. Soot includes a variety of matter such as elemental carbon, heavy hydrocarbons, hydrated sulfuric acid, and other large molecules, and are generally associated with non-optimal combustion. Soot can be reduced by increasing combustion and/or exhaust temperatures, or by providing more oxygen to promote oxidation of the soot particles. Unfortunately, measures which reduce NOx tend to increase soot emissions, and measures which reduce soot tend to increase NOx emissions, resulting in what is often termed the “soot-NOx tradeoff”.
At the time of this writing, the diesel engine industry is facing stringent emissions legislation in the United States, and is struggling to find methods to meet government-imposed NOx and soot restrictions. One measure under consideration is use of exhaust after-treatment (e.g., particulate traps) for soot emissions control in both heavy-duty truck and automotive diesel engines. However, in order to meet mandated durability standards (e.g., 50,000 to 100,000 miles), the soot trapped must be periodically re-burned. This requires considerable expense and complexity, particularly since additional fuel must often be mixed and ignited in the exhaust stream in order to burn off the accumulated soot deposits.
Apart from studies directed to after-treatment, there has also been intense interest in the more fundamental issue of how to reduce NOx and soot generation from the combustion process. Studies in this area relate to shaping combustion chambers and/or modifying the timing, rate, and/or shape of the fuel injection to attain desired effects. One field of study relates to injection premixing methodologies, wherein the object is to attain more complete mixing of fuel and air in order to simultaneously reduce soot and NOx emissions. In diesel engines, the object of premixing methodologies is to move away from the diffusion burning mechanism which drives diesel combustion, and instead attempt to attain premixed burning. In diffusion burning, the oxidant (fuel) is provided to the oxidizer (air) with mixing and combustion occurring simultaneously. The fuel droplets within an injected spray plume have an outer reaction zone surrounding a fuel core which diminishes in size as it is consumed, and high soot production occurs within the fuel-rich spray core. In contrast, premixed burning mixes fuel and air prior to burning, and the more thorough mixing results in less soot production. Premixing may be performed by a number of different measures, such as by use of fumigation (injection of vaporized fuel into the intake airstream prior to its entry into the engine), and/or direct injection of a fuel charge relatively far before top dead center (TDC) so that piston motion and convection within the cylinder, in combination with the relatively long period of time before TDC (and ignition), result in greater mixing.
One promising diesel premixing technology of this nature is HCCI (Homogeneous Charge Compression Ignition), which has the objective of causing initial ignition of a lean, highly premixed air-fuel mixture at or near top dead center (TDC). An extensive discussion on HCCI and similar premixing techniques is provided in U.S. Pat. No. 6,230,683 to zur Loye et al., and U.S. Pat. Nos. 5,832,880 to Dickey and 6,213,086 to Chmela et al. also contain useful background information. The charge is said to be “homogeneous” in HCCI because it is (at least theoretically) highly and evenly mixed with the air in the cylinder. Ignition is then initiated by autoignition, i.e., thermodynamic ignition via compression heating. The objective of HCCI is to use autoignition of the lean and homogeneous fuel-air mixture to provide a uniform non-diffusion (or minimized diffusion) burn, resulting in significantly lower combustion chamber temperatures and diminished NOx production (which thrives at high temperature), as well as lower soot production owing to enhanced mixing and the resulting reduction or elimination of fuel-rich regions. In contrast, a richer mixture (such as that necessary for flame propagation from the spark in an SI engine) will burn more quickly at greater temperature, and therefore may result in greater NOx production.
Another example of a methodology for modifying fuel injection to attain desired combustion results is presented in U.S. Pat. No. 6,526,939 to Reitz et al., wherein multiple injections are used during an engine cycle rather than a single injection (preferably starting at or near the end of the compression stroke), and wherein successive injections experience an increase in their injection pressure (injection rate) and a decrease in the fuel volume injected. This injection scheme is described as promoting lower emissions, possibly owing to better mixing and/or owing to a more controlled heat release from the injected fuel (and thus lower peak temperatures and lower NOx production). Further emissions reductions can be attained with use of EGR or other exhaust after-treatment methodologies.
Despite the advances offered by the foregoing methods, it would be useful to have additional and/or improved emissions reduction methods available, particularly in view of the ever-increasing need for decreased emissions.